ABSTRACT Radiation therapy plays a key role in the loco-regional management of patients with high- grade STS. Currently, T2-weighted Magnetic Resonance Imaging (MR) is the modality of choice in the treatment planning of STS. Unfortunately, peri-tumoral edema in soft STS can extend up to 4 cm from the T2 gross tumor. This is the reason why the current radiation therapy oncology group (RTOG) consensus a volume that exceeds MRI defined tumor dimension by 3.5 cm longitudinally and 1.5 cm radially. Additional T1-weighted contrast enhanced suspicious edemas are usually added to the expansion that can be further problematic given the extensive T1 abnormalities associated with many of these lesions and extra cost of T1-weighted dynamic study. In some cases, high dose radiation can be difficult in the management of STS because the morbidity would be unacceptable or nearby critical organs make extended radiation impossible. Thus, it becomes clear why the morbidity associated with STS is so prohibitive. Our proposal tackles this very important question. Instead of standard anatomical based MRI delineation, metabolic oncologic imaging with FDG-PET or MRS has been shown to higher sensitivity and specificity. By using the strengths of combined PET and MRI/MRS, we propose to define a more accurate delineation of suspicious peri-tumoral edema for preoperative radiation therapy Clinical Target Volume (CTV). If successful, the impact of our work will be substantial as it would reduce the CTV of radiation therapy, and the associated morbidity, without compromising the treatment efficacy and while allowing greater specificity is assessing response to treatment that can be achieved with PET or MRS alone.